Broad-band variable-wavelength laser beam generator

ABSTRACT

There is provided a broadly tunable laser beam generator serving as a laser beam source which utilizes a nonlinear optical effect of a silica (glass) fiber and which is broadly tunable in the near-infrared region, having ultra-broad tunability which has not been easily achieved by known tunable lasers, and generating coherent light which can be continuously swept over the entire wavelength region with a simple mechanical operation of a single wavelength selecting element and which is emitted in a constant direction independent of its wavelength.  
     The laser beam source which utilizes a nonlinear optical effect of a silica optical fiber ( 8 ) and which is broadly tunable in the near-infrared region has ultra-broad tunability and generates coherent light which can be continuously swept over the entire wavelength region with a single wavelength selecting element ( 10 ).

TECHNICAL FIELD

[0001] The present invention relates to a broadly tunable laser beamgenerator.

BACKGROUND ART

[0002] In the fields of optics in general including spectroscopy andoptical communications, a light source offering a broadband tunabilityhas been eagerly desired. Although the term “broadband” is generic anddefined only vaguely, it here points to a band from 800 to 1800 nm thatis of importance in the fields of spectroscopy and opticalcommunications. In many applications, an incandescent lamp or the likecombined with a spectrometer has been the optimal choice as a broadlytunable light source; the intensity of light remains no more than a fewtens of μW per 1 nm, and even worse, convergence of light is not as goodas that of coherent light like laser radiation. This is solelyattributable to incoherence of light generated in an incandescent lamp.

[0003] Driven by the same desire, several lasers as a coherent lightsource having a fair tunability have been developed. In fact, some ofthem have been put in practical use in the many and expanding fields ofapplication.

DISCLOSURE OF INVENTION

[0004] Among those tunable lasers developed so far, a Ti-sapphire laseris a well-known example, which is capable of tuning 300 nm continuously.The widest tuning range is provided by an optical parametric oscillator(OPO) laser, which can go from the ultraviolet all the way to theinfrared with the aid of the third-order nonlinearity in an opticalcrystal of macroscopic size. However, there are still problemsassociated with the OPO: wavelength sweep is not easy in the pump-lightdegenerate region, an oscillation linewidth is broad (>1 nm), anlight-emitting direction varies with wavelength unless appropriate anglecompensation is exercised, and very high-power pump light is required.

[0005] Meanwhile, fiber lasers utilizing an optical fiber as a broadbandgain medium have been already on the market, such as a tunable Ramanlaser. For these commercial fiber lasers, a broad wavelength tuningrange 1073 to 1600 nm is predicted according to literature values oftunability due to the third-order nonlinearity. However, the tuningrange is deliberately limited to within 100 nm in practice as there is atrade-off between the broad tunability and small spectral linewidth. Itis sometimes even required to designate a particular wavelength and thusthe potential broad tunability is not taken advantage of.

[0006] In addition, it has been taken for granted that as long asstimulated Raman scattering is concerned, low-energy light (Stokes lightor down-converted light) is what is mainly generated, and up-convertedlight (anti-Stoke light) is very unlikely and thus wavelength tuning isnot allowed in the pump-light degenerate region as well as at the highenergy side.

[0007] In view of the above-mentioned circumstances, it is an objectiveof the present invention to provide a broadly tunable laser beamgenerator serving as a laser beam source which utilizes a nonlinearoptical effect of a silica (glass) optical fiber and which is broadlytunable in the near-infrared region, offering ultra-broadband tunabilitythat has not been easily achieved by the known tunable lasers, andgenerating coherent light whose direction of emission does not depend onwavelength and which scans the entire wavelength region with a singlewavelength selecting element.

[0008] In order to achieve the above objective, according to the presentinvention,

[0009] [1] a silica optical fiber having birefringence is used as a gainmedium, and a non-linear optical effect of the fiber (stimulated Ramanscattering and stimulated parametric four-lightwave mixing) is utilized;

[0010] [2] by using a pulse laser having an oscillating wavelengthshorter than a cut-off wavelength as pump light, a gain spectrumcontinuously extending in the degenerate region in the vicinity of thepump light and also over both a shorter wavelength (anti-Stokes light)region and a longer wavelength (Stokes light) region with respect to awavelength of the pump light is generated;

[0011] [3] a diffraction grating is used as a broadly tunable singlewavelength selecting element;

[0012] [4] a collinear emitting configuration for generating a laserbeam from an end of the fiber is employed; and

[0013] [5] a laser gain is obtained by passing light through the fibertwice without requiring pulse-tuning adjustment.

[0014] A tunable range of 800 to 1800 nm according to the presentinvention covers even 210 THz on an energy scale. Such a light source(laser) easily tunable across a broadband with direction of emissionremaining collinear and configured in a simple geometry is unparalleled.

[0015] Here the term “collinear” means that light in the laseroscillator has a particular direction of emission regardless ofwavelengths: light always emits from the end of the fiber so that it isoriented towards a fixed direction in space. This is well contrastedwith the case of OPO. Since direction of emission of a laser beamgenerated in an OPO crystal often varies with wavelength as it isscanned, it is required to introduce a second crystal to compensate forthe angular departure. This results in a complicated structure andoperation. Even worse, the OPO still cannot get rid of the problem thatthe laser beam is not perfectly collinear.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 is a diagrammatic view of a broadly tunable laser beamgenerator according to an embodiment of the present invention.

[0017]FIG. 2 is a diagram illustrating tuning characteristics accordingto the embodiment of the present invention.

[0018]FIG. 3 is a diagram of single wavelength selection according tothe embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

[0019] An embodiment of the present invention will be described indetail.

[0020]FIG. 1 is a diagrammatic view of a broadly tunable laser beamgenerator according to the embodiment of the present invention, FIG. 2is a diagram illustrating tuning characteristics according to theembodiment of the present invention, wherein the horizontal and verticalaxes respectively indicate an oscillating wavelength (in nm) and arelative optical-output intensity (in dB). FIG. 3 demonstrates specificexamples of single-wavelength selection, wherein the horizontal andvertical axes respectively indicate a wavelength (in nm) and a relativeoptical-output intensity (on an arbitrary scale), and the three examplesof wavelength selection shown have been shifted along the vertical axisfor clarity. The peak at the extreme left indicates a pump laser beam asa reference.

[0021] In FIG. 1, the reference numeral 1 denotes a pump pulse laser, 2denotes directions of polarized light of the pumping pulse laser 1, 3denotes a polarization beam splitter, 4 denotes a quarter-wave plate, 5denotes a direction of circularly polarized light generated by thequarter-wave plate 4, 6 denotes an output-coupling half-mirror, 7 and 9denote collective collimator lenses, 8 denotes a silica optical fiberserving as a gain medium, and 10 denotes a diffraction grating servingas a single-wavelength selecting element.

[0022] Although the polarization beam splitter 3 and the quarter-waveplate 4 constitute an isolator to remove unwanted pump reflection thatis fedback to the pump laser 1, they may be replaced with an alternatephotonic device having the same function as them. In addition, even whenthe polarization beam splitter 3 and the quarter-wave plate 4 areremoved, the generation characteristics of a broadband tunable laserbeam according to the present invention is not affected.

[0023] A pulse beam emitted from the pump pulse laser 1 (with anoscillating wavelength of 1064 nm) (it may be a Q-switched ormode-locked pulse laser which can generate a high-power pulse beam, andmore preferably, it may be the one with a pulse beam having an outputintensity of 1 kW or more and having oscillating wavelengths shorterthan a cut-off wavelength of the single-mode silica optical fiber 8 byabout 10 to 20% in addition to 1064 nm) passes through theoutput-coupling half-mirror 6 and is incident on the silica opticalfiber 8 serving as a gain medium via the collective collimator lens 7.

[0024] The silica optical fiber 8 is a single mode fiber having abirefringent characteristic, a cut-off wavelength of 1100 nm to 1300 nm,and a length of 100 m to 400 m. When a polarization-preserving fiber isused, the fiber length can be made even shorter. Also, in order toachieve this purpose, even a refractive-index-gradient-type multimodefiber (graded-index fiber) for communications, having 50 and 62.5 μmcore diameters, can be used instead of a single mode fiber, aside fromthe fact that its output-light intensity characteristics is differentfrom those of the single mode fiber.

[0025] While pulse light is propagating through the silica optical fiber8, a nonlinear optical effect of silica glass allows stimulated Ramanscattering and stimulated parametric four-wave mixing. Phase-matchingconditions of the former are automatically satisfied with dispersion ofphonons, and those of the latter is compensated for with modaldispersion of wavelengths shorter than the cut-off wavelength, caused bya waveguide.

[0026] The light emitted from the other end of the silica optical fiber8 is converted into collimated light by the collective collimator lens 9and is incident on the diffraction grating 10, and only first-orderdiffracted light is selectively fed back to the silica optical fiber 8.Preferably, the diffraction grating (single-wavelength selectingelement) 10 maintains a high reflectance in a working wavelength band inorder to improve a feedback efficiency.

[0027] To be specific, with 830 to 1000 nm in blaze wavelength, 600 to1200 lines/mm, 10×10 mm² to 25×25 mm², and gold or aluminum coating, thediffraction grating offers an excellent result. Alternatively, acombination of a narrow-linewidth band-pass filter and a totalreflection mirror may be used.

[0028] The fed back light having a specific wavelength is amplified byobtaining a gain while propagating through the fiber twice in bothdirections, is emitted from the one end of the fiber on which it wasfirst incident, is converted into collimated light by the collectivecollimator lens 7, and is emitted as an output light beam by theoutput-coupling half-mirror 6. It is emitted in a constant directionindependent of its wavelength, that is, it becomes collinear.

[0029] Although the highest output is in principle obtained when thereflectance of the output-coupling half-mirror 6 is 50%, it may be 10 to50%.

[0030] Measured results of light outputs in FIG. 2 are shown by relativeintensities normalized by one of the measured results of 3 mW at 1178nm. An absolute output of a laser according to the present invention canbe increased substantially in proportion to a pump intensity.

[0031] As single-wavelength selection characteristics is shown in FIG.3, with the above-described structure, a linewidth (full width at halfmaximum) of 1 nm or less is obtained, and the linewidth can be furthernarrower in reverse proportion to the grooved line density of thediffraction grating used for feedback of light.

[0032] Meanwhile, the present invention is not limited to the foregoingembodiment, and, since a variety of modifications are possible based onthe spirit of the present inventions, these modifications shall not beexcluded from the scope of the present invention.

[0033] As described above in detail, according to the present invention,a laser beam generator (coherent light source) which is tunable in abroadband can be constructed. A tunable region of 800 to 1875 nm isuseful in a very wide range and immediately available as an excitationlight source for spectroscopy including a solid state matter, and alsocovers all wavelength bands of 830 nm, 1310 nm, 1480 nm, and 1500 to1600 nm which are important in optical communications. Especially, inthe latter case, although an efficiency of coupling the light source toa fiber is vital, since a laser beam is emitted from the fiber itself,its coupling efficiency is about twice as much as that of spatial beamcoupling, and also it is advantageous from the view point of maintenancebecause of no loss caused by scattering.

[0034] Also, the generator has a simpler structure than an opticalparametric oscillator (OPO), collinearly emits light independent of itswavelength, and also easily performs wavelength selection.

INDUSTRIAL APPLICABILITY

[0035] A broadly tunable laser beam generator according to the presentinvention is suitable as a coherent, broadband tunable light source of afiber-emitting-type in the field of spectroscopy and as a coherenttunable light source having a fiber-pigtail specification in the fieldof optical communications.

1. A broadly tunable laser beam generator serving as a laser beam sourcewhich utilizes a nonlinear optical effect of a silica optical fiber andwhich is tunable in a broadband in the near-infrared region, havingultra-broad tunability, and generating coherent light which can be sweptover the entire wavelength region with a single wavelength selectingelement and which has a collinear emittance directivity, wherein thesilica optical fiber is a single mode fiber in which birefringenceremains and whose cut-off wavelength lies preferably from 1100 to 1300nm, and wherein, by coupling a pulse laser beam source with an end faceof the fiber so as to input a pulse laser beam having a short wavelengthvery close to the cut-off wavelength into the end face of the fiber aspump light, stimulated Raman scattering and stimulated parametricfour-lightwave mixing occur in the single fiber so as to provide a gainspectrum continuously extending over both shorter and longer wavelengthregions with respect to a wavelength of the pump light, and consequentlyto provide ultra-broadband tunability.
 2. The broadly tunable laser beamgenerator according to claim 1, wherein the ultra-broad tunability liesfrom 950 nm to 1875 nm.
 3. (Deleted)
 4. The broadly tunable laser beamgenerator according to claim 1, wherein the tunability is completelycontinuous to a degenerate region close to the pump light.